scratchy audio over hdmi

I'm having some trouble with hdmi audio on pcsx. This is on Ubuntu 12.04 LTS, with pcsxr 1.9.92-3build1 from the software center, and the machine is a Zotac ZBOX with an AMD E-350 cpu and a Radeon HD 6310 gpu with the latest Catalyst drivers (have to double
check exact version).

The problem is I get scratchy audio over hdmi. When there's no sound in game, there's no scratches, and as the game sound gets louder, the scratches get louder too. It's sort of crackling and popping, or crunchy.

The audio sounds fine from the headphone jack (analog). I also have zsnes and fceux running with clean audio over both hdmi and analog. I'm testing with Castlevania:SOTN.

I think the problem has to do with sample rate, because when I started testing fceux I was getting the same symptom (scratchy over hdmi, clean over analog) until I changed the sample rate to 32kHz (to match zsnes). Unfortunately, I can't do much for tests with
pcsx in this regard -- there's no sample rate selection.